A Fabry-Perot etalon filter has two partially reflective mirrors, or surfaces, facing each other and separated by a certain gap which forms a cavity. The spectral characteristics of an etalon filter are generally determined by the reflectivity and gap spacing of the mirrors or surfaces. The Fabry-Perot principle allows a wideband optical beam to be filtered whereby only a spectral passband is transmitted out of the filter. Tuning of the center wavelength of the spectral passband is achieved typically by varying the effective cavity length (spacing).
It is advantageous in certain instances to provide a higher level of faltering, resulting in a more narrow wavelength passband, by effecting two or more passes of the optical beam through a Fabry-Perot cavity (etalon filter). This can obviously be achieved by combining two etalon filters through which the beam is passed in series. However, since the spacing of the cavity is so important, small deviations in the tuning of two separate cavities may result in an unsatisfactory definition of the peak transmission wavelength. Therefore, it is preferable to effect two passes through the same Fabry-Perot (F-P) cavity.
Such an approach has been described for instance in the U.S. Pat. No. 5,283,845 issued Feb. 1, 1994 to J. W. Ip. In one of the embodiments, a transmitted output beam is fed back into the etalon filter and is thus filtered twice through the same F-P cavity through an optical fiber.
Although this arrangement is advantageous over single pass devices, it has associated coupling loss due to the need to couple twice to the optical fiber. Furthermore, the mirror separation may be slightly different at the two beam locations, resulting in differences in peak transmission wavelengths. Thus there is a need for further improvement of the performance of multi-pass etalon filters.